Regulation of the resistance and resilience of periparturient ewes to infection with gastrointestinal nematode parasites by dietary supplementation

Control of gastrointestinal nematode (GIN) parasites has traditionally been based on the premise of chemotherapeutic control with the resultant consequence being the development of anthelmintic resistance by GIN. An alternative premise to the management of GIN parasites is to improve the resistance and resilience of sheep to GIN infection by manipulating the nutritional environment at key periods of a sheep's life. One key period, is the periparturient phase in the reproductive cycle when ewes experiences a temporary loss, or diminution, of immunity to GIN parasites. This phase is associated with an increased requirement for both metabolisable energy (ME) and metabolisable protein (MP). The increased requirement being greater for MP. The loss of immunity to GIN is associated with a diminished cell-mediated immune response in the gut mucosa, the magnitude of which can be regulated by protein nutrition, or more specifically the supply of MP. Experimental studies that have increased MP supply, in housed and grazing periparturient ewes, have demonstrated significant improvements in resistance to GIN infection. Positive gut immune responses to an increased MP supply are believed to occur because the increased MP supply counters the combined pathological consequences of GIN infection per se and the host's immune response, namely a net loss of amino acids. Susceptibility to GIN infection in the periparturient ewe may also be a function of a low priority for use of MP toward immune function but evidence from young animals suggests a disproportionate partitioning of amino acids to the gastrointestinal tract during GIN infection. It is proposed that the priorities for use of MP that predispose the ewe to GIN infection are altered following infection to favour a gut immune response. Despite being under the regulation of MP supply and genetic selection, the loss of immunity during the periparturient period cannot be fully restored by either approaches suggesting that other unidentified factors are involved in the periparturient loss of immunity. Resilience to GIN infection is responsive to both ME and MP supply. The practice of supplementing periparturient ewes to increase MP and ME supply, in order to enhance resistance and resilience to GIN infection, is gaining favour with graziers within some regions of Australia. Other benefits (e.g. increased reproductive rates) arise from such supplementation strategies that improve the cost effectiveness of this approach.